The practical utility of many drugs having potentially useful biological activity is often stymied by difficulty in delivering such drugs to their targets. Compounds to be delivered into cells must generally be delivered from a largely aqueous extracellular environment and then penetrate a lipophilic cell membrane to gain entry to the cell. Unless the substance is actively transported by a specific transport mechanism, many molecules, particularly large molecules, are either too lipophilic for practical solubilization or are too hydrophilic to penetrate the membrane.
A segment of the HIV Tat protein consisting of amino acid residues 49-57 (Tat 49-57, having the sequence RKKRRQRRR) (SEQ ID NO.:11) has been used to deliver biologically active peptides and proteins to cells (e.g. Barsoum et al., 1994, PCT Pubn. No. WO 94/04686). Tat (49-60) has been used to enhance delivery of phosphorothioate oligonucleotides (Astriab-Fisher, Sergueev et al. 2000; Astriab-Fisher, Sergueev et al. 2002). Reverse Tat, or rTat(57-49) (RRRQRRKKR) (SEQ ID NO.:12), has been reported to deliver fluorescein into cells with enhanced efficacy compared to Tat (49-57) (Wender, Mitchell et al. 2000; Rothbard, Kreider et al. 2002). Rothbard and Wender have also disclosed other arginine-rich transport polymers (PCT Pubn. No. WO 01/62297; U.S. Pat. No. 6,306,993; US Patent Appn. Pubn. No. 2003/0032593).
Oligonucleotides are one class of potentially useful drug compounds whose delivery has often been an impediment to therapeutic use. Phosphorodiamidate-linked morpholino oligomers (PMOs, see e.g. Summerton and Weller, 1997) have been found more promising in this regard than charged oligonucleotide analogs such as phosphorothioates. The PMOs are water-soluble, uncharged or substantially uncharged antisense molecules that inhibit gene expression by preventing binding or progression of splicing or translational machinery components. PMOs have also been to shown to inhibit or block viral replication (Stein, Skilling et al. 2001; McCaffrey, Meuse et al. 2003). They are highly resistant to enzymatic digestion (Hudziak, Barofsky et al. 1996). PMOs have demonstrated high antisense specificity and efficacy in vitro in cell-free and cell culture models (Stein, Foster et al. 1997; Summerton and Weller 1997), and in vivo in zebrafish, frog and sea urchin embryos (Heasman, Kofron et al. 2000; Nasevicius and Ekker 2000), as well as in adult animal models, such as rats, mice, rabbits, dogs, and pigs (see e.g. Arora and Iversen 2000; Qin, Taylor et al. 2000; Iversen 2001; Kipshidze, Keane et al. 2001; Devi 2002; Devi, Oldenkamp et al. 2002; Kipshidze, Kim et al. 2002; Ricker, Mata et al. 2002).
Antisense PMO oligomers have been shown to be taken up into cells and to be more consistently effective in vivo, with fewer nonspecific effects, than other widely used antisense oligonucleotides (see e.g. P. Iversen, “Phosphoramidite Morpholino Oligomers”, in Antisense Drug Technology, S. T. Crooke, ed., Marcel Dekker, Inc., New York, 2001). However, further enhancement in uptake and antisense efficacy is desirable in order to fully explore their potential.